Disk array systems devices which allow great volumes of data to be transferred in and out and from many disks at high transfer speed and with relatively good reliability have been in use for many years. These disk array systems were classified as Redundant Arrays of Inexpensive Disks (RAID) 1 to 5 by Dr. David A. Patterson et al. of the University of California (see ACM SIGMOD Conference, Chicago, Ill., Jun. 1-3, 1988, P.109-166). The present invention relates primarily to RAID-4 and RAID-5, which will now be discussed.
It is known in the art that the reliability of the RAID-4 and the RAID-5 systems is provided by a parity or redundant data which is created from the combination of data stored in the same memory block location of each of the disk devices. The data which create the parity data are formed into a group called a parity group. The parity data of each parity group is stored in each of the disk devices in the array and are included in the parity group.
One of the characteristics of the RAID-4 and the RAID-5 systems is that each time data of a particular parity group is altered, the parity data of that parity group is recalculated and updated so that the redundancy of the parity data is maintained. This requires the disk control device to not only access the memory block of a disk containing the desired data, but also the memory block of the disk containing the corresponding parity data, each time the data is accessed from the disk array. Consequently, this causes a delay in operation, since the disk control device for the RAID-4 and the RAID-5 systems must always access an additional memory block each time data is accessed. In other words, the inherent characteristics of the RAID-4 and the RAID-5 systems result in reduced access speed during the reading and writing processes. Thus, there is a need for a disk control device for the RAID-4 and the RAID-5 systems which is able to increase the access speed.
Accordingly, it is one object of this invention to provide a new and improved disk control device for the RAID-4 and the RAID-5 systems.
Another object is to provide a new and improved disk control device having increased access speed for use with the RAID-4 and the RAID-5 systems.
Yet another object of this invention is to provide a new and improved disk control device for the RAID-4 and the RAID-5 systems which does not access the parity data each time the nonparity data is accessed.
Still another object of this invention is to provide a new and improved disk control device for the RAID-4 and the RAID-5 systems which holds the parity data in a cache until all the nonparity data of the corresponding parity group are transferred out of the cache.